FIG. 2 is a plan view showing the surface of an active layer of a conventional laser with a wide active region aiming at a single transverse mode. In FIG. 2, reference numeral 21 designates a fundamental mode propagation region, reference numeral 22 designates a mode magnifying region, reference numeral 15 designates a partially reflecting coating having low reflectivity, reference numeral 16 designates output light and reference numeral 25 designates a highly reflecting coating.
In the laser shown in FIG. 2, since a light waveguide for the fundamental mode propagation region 21 has a small width w.sub.1 of 3 .mu.m.+-.2 .mu.m, the fundamental mode is only propagated, in principle. In this case, the reason why the width w.sub.1 of the light waveguide has a range in its established value is because the thickness of the active layer or the like should be taken into consideration as a factor other than in addition to the width of the light waveguide. For example, when the active layer is thin, even if the width of the light waveguide is to some degree large, only fundamental mode oscillation is allowed. Or when the difference in refractive index between the light waveguide part and the parts on both sides of its is small, even if the width of the light waveguide is large, only fundamental mode oscillation is allowed. The width of the light waveguide of the mode magnifying region 22 gradually increases from a connection to the light waveguide of the fundamental mode propagation region 21 to the facet. In this case, if the divergent angle .theta..sub.1 of the waveguide is too large, light from the fundamental mode propagation region 21 is not magnified and then it reaches the facet as it is, so that high power light output can not be implemented. Alternatively, when the length L.sub.1 of the mode magnifying region 22 is too long, the possibility that higher order modes of oscillation may occur while the laser beam propagates in the region 22 is increased, with the result that fundamental mode oscillation can not be obtained. In order to obtain the fundamental mode oscillation high power light output, the divergent angle .theta..sub.1 is 10.degree. or less and the length L.sub.1 of the mode magnifying region 22 is 10 to 100 .mu.m. Therefore, the width w.sub.2 of the light waveguide on the emitting facet is 10 to 20 .mu.m.
The fundamental mode generated in the fundamental mode propagation region 21 is amplified and magnified in the mode magnifying region 22 and then taken out as the output light 16 through the partially reflecting coating 15. The light partially reflected by the partially reflecting coating 15 reaches the opposite facet through the mode magnifying region 22 and the fundamental mode propagation region 21 and is reflected by the highly reflecting coating 25 again to contribute to oscillation. In this case, the reflectivity of the facet on which the partially reflecting coating 15 is applied is 3 to 18% and reflectivity of the facet on which the highly reflecting coating 25 is applied is 60 to 98%. FIG. 5 is a graph showing the difference in laser light output due to the reflectivity of the emitting facet. In the figure, the ordinate shows light output and the abscissa shows injection current. Reference character A designates the characteristic of a laser having a high reflectivity emitting facet and reference character B designates the characteristic of a laser having a low reflectivity emitting facet. As can be seen from this figure, the reflectivity on the emitting facet may be selected according to the desired light output.
According to the conventional laser, a high power light output is achieved by increasing the width of the output light 16 as described above. Actually, when a crystal of AlGaAs is used, oscillation in an 0.8 .mu.m wavelength band in a single transverse mode at a power of 100 mW or more has been observed.
In addition, FIG. 3 is a plan view showing an active layer surface of another conventional semiconductor laser device with a high output. In FIG. 3, the laser active region 11 is arranged at the center of the active layer in the direction of its width and a non-reflecting coating 12 whose reflectivity is approximately 3% is applied to the whole emitting facet. An partially reflecting partial coating 34 whose reflectivity is approximately 45% is applied to the center of the non-reflecting coating 12 in the direction of the width of the laser active region 11. A highly reflecting coating 25 whose reflectivity is approximately 98% is applied to the whole opposite facet of a resonator. Reference numeral 16 designates output light. In addition, the length of the resonator is approximately 500 .mu.m.
FIGS. 4(a)-4(d) are graphs of intensity distribution of each mode when the front facet is viewed from the inside of the semiconductor laser device shown in FIG. 3. In FIGS. 4(a)-4(d), reference numeral 41 designates the intensity distribution of the fundamental mode, reference numeral 42 designates the intensity distribution of the primary mode, reference numeral 43 designates the intensity distribution of the secondary mode, and reference numeral 44 designates the intensity distribution of the tertiary mode.
Next, operation thereof will be described. The light generated in the laser active region 11 of the semiconductor laser device is mostly reflected by the highly reflecting coating 25 on the rear facet and amplified in the laser active region 11 and then reaches the non-reflecting coating 12 and the partially reflecting partial coating 34. The light which reaches the non-reflecting coating 12 is not reflected and becomes the output light 16. In addition, the light which reaches the partially reflecting partial coating 34 is partially transmitted and then becomes the output light 16 but the rest of the light is reflected and amplified in the laser active region 11 again and then reaches the non-reflecting coating 12 to contribute laser oscillation. In addition, although it is illustrated that the light reflected by the partially reflecting partial coating 34 and the highly reflecting coating 25 is considerably magnified in the resonator and most of the output light 16 is output through the non-reflecting coating 12 in FIG. 3, this magnification is small in fact and most of the output light 16 is output through the partially reflecting partial coating 34.
Next, the principle of fundamental transverse mode oscillation in accordance with this prior art laser device will be described. The width of the active region in this laser is large enough to allow a higher order mode of oscillation. In this case, when the frequency is fixed, spatial expansion in the transverse direction varies with the degree of the mode propagating in the semiconductor laser device. More specifically, as shown in FIGS. 4(a)-4(d) when the spatial width is fixed at the width of the partially reflecting partial coating 34, the energy within the width is the largest in the fundamental mode and becomes smaller as the degree of the mode increases. Therefore, since effective reflectivity is the highest in the fundamental mode, oscillation at the fundamental mode is most easily performed. In fact, a partially reflecting coating of AIGaAs with a width of 20 .mu.m and reflectivity of approximately 40% is applied to an active region having a width of 150 .mu.m for wavelength of 0.8 .mu.m, oscillation in only the fundamental mode with a light output of 300 mW or more is observed.
However, in the prior art laser device shown in FIG. 2 the mode magnifying region 22 is not perfect because of small disturbances such as crystalline defects in the mode magnifying region formed when the semiconductor laser device is formed or differences in current gain within the mode magnifying region, so that the propagation mode is influenced in the mode magnifying region and a higher order mode oscillation occurs and is propagated. Thus, when the light output exceeds around 100 mW, the imperfections of the mode magnifying region 22 are gradually manifested together with lack of uniformity of a current, so that oscillation is likely to occur in the higher order mode.
In addition, according to the laser shown in FIG. 3, since the mode in the transverse direction is controlled only by a spatial film having no distribution in refractive index, there is almost no disturbance at the time of forming the waveguide as shown in FIG. 2, so that higher order mode oscillation is not likely to occur. However, as shown in FIG. 5, when the reflectivity of the facet is increased, laser oscillation can easily arise, but the output power is smaller than that when the reflectivity on the facet is low, which is the problem of the laser shown in FIG. 3. More specifically, when the reflectivity of the partial coating 34 is increased, oscillation can easily occur but the light is difficult to output. Therefore, this structure is effective in fundamental mode oscillation of a laser having a wide active region. However, it is not always effective as final means for high power light output.